The present invention is directed to the method of manufacturing drilling bits and more particularly involves those types of bits disclosed in a companion application Ser. No. 156,883, filed June 5, 1980 by John D. Parrish entitled "Large Diameter Oil Well Drilling Bit", now U.S. Pat. No. 4,369,849, issued Jan. 25, 1983. A second, co-pending application, Ser. No. 156,715, filed June 5, 1980 by John D. Parrish entitled "Method of Manufacturing Large Diameter Oil Well Drilling Bit" also relates to the present application. These two companion applications are hereby incorporated herein by reference. This disclosure illustrates an improvement bit and method of manufacturing said bit over those disclosed in the above-incorporated applications.
Large diameter bits, generally termed "tophole" bits generally comprise either four cutter bits or else they are tricone bits such as that disclosed herein. The general method of manufacturing large diameter tri-cone bits is in utilizing the segmented arc construction method. In this method, three 120 degree lug sections are welded together to form a generally cylindrical bit body having downwardly extending leg assemblies. The three sections are usually forged and machined to form the 120 degree lugs and then the cutter assemblies are formed and placed on the lugs prior to welding the three lugs to form the single bit. Because of the size of the tophole bits, minor variations and tolerances in angles of assembly and the alignments of the three lug sections result in substantial final errors in the bit specifications and dimensions. Because of the roughness and inaccurateness of the three arcuate lug sections machining of the mating surfaces is difficult to control within tolerance. When the three lug sections are welded together to form the final bit structure, the cutters are generally not aligned axially or radially within the desired bit specifications.
The aforementioned incorporated companion applications disclose bits and their methods of manufacture which are significant improvements over the triple segment bit construction method of the conventional bits. In the aforementioned companion applications, a unitary bit body is formed of cast metal and three separate shoulders or recesses are machined individually into the bit body to receive pre-assembled lug assemblies. Each of the recesses comprises a generally right angular shoulder or shelf area to receive the mating right angular end of each particular lug assembly. Generally, these right angular shoulders are formed individually by horizontal milling techniques. After the first two lug assemblies have been welded in place on the unitary body, the third lug assembly is placed in proper alignment and shimmed and welded in place. The third lug assembly is difficult to weld in place because of the restricted access to the weld area due to the placement of the first two lug assemblies. Because of the close intermesh of the three conical cutters, welding must be performed through the side of the bit through the openings between the three downwardly extending lugs. Generally, the welding of the first two lug assemblies is not as difficult as that of the third assembly because of the easier access to the first two lug assemblies through the area where the third assembly must later be placed.
The present invention eliminates the need for three separate horizontal milling operations to prepare the unitary bit body and also eliminates the highly restricted welding access feature of the aforementioned incorporated disclosures. The present invention also overcomes the inaccurate assembly methods required in the three arcuate segment construction of conventional tri-cone bits. These advantages are overcome by providing a single unitary bit body which may be machined to receive the three lug assemblies in a single pass rotary milling operation which also provides greatly increased access to welding the three lug assemblies to the body.